Method dfs

python/0130-surrounded-regions.py:6–10

(r, c)

        flag = set()

        def dfs(r, c):
            if not(r in range(rows) and c in range(cols)) or board[r][c] != 'O' or (r, c) in flag:
                return
            flag.add((r, c))
            return (dfs(r + 1, c), dfs(r - 1, c), dfs(r, c + 1), dfs(r, c - 1))

        # traverse through the board
        for r in range(rows):